This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/DE01/00376 which has an International filing date of Jan. 31, 2001, which designated the United States of America and which claims priority on German Patent Application No. 100 04 614.2 filed Feb. 3, 2000, the entire contents of which are hereby incorporated by reference.
The invention generally relates to a pressure sensor for sensing the pressure of a fluid, in particular, a solution of reducing agent for exhaust gas post-treatment in an internal combustion engine.
The nitrogen oxide emissions of an internal combustion engine which operates with an excess of air, in particular a diesel internal combustion engine, can be reduced using the SCR (Selective Catalytic Reduction) method to form atmospheric nitrogen (N2) and the water vapor (H2O). Gaseous ammonia (NH3), ammonia in aqueous solution or urea in aqueous solution is used as the reducing agent. The urea serves here as an ammonia carrier, and using a metering system upstream of a hydraulic catalytic converter it is injected into the exhaust system, converted there into ammonia by way of hydrolysis, and then the nitrogen oxides are in turn reduced in the actual SCR or DENOX catalytic converter.
Such a metering system has, as essential components, a reducing agent container, a pump, a pressure regulator, a pressure sensor, a metering valve and the necessary connecting hoses. The pump feeds the reducing agent stored in the reducing agent container to the metering valve by way of which the reducing agent is injected into the exhaust gas stream upstream of the hydrolysis catalytic converter. The metering valve is actuated by way of signals of a control device in such a way that a specific quantity of reducing agent which is necessary at a given time is fed to the internal combustion engine as a function of operating parameters (DE 197 43 337 C1).
An advantage of the substances, for example urea, which are present in aqueous solutions and which release ammonia is that the storage, handling, feeding and metering can be dealt with relatively easily in technical terms. A disadvantage of these aqueous solutions is that there is a risk of freezing at specific temperatures as a function of the concentration of the dissolved substance.
32% urea solution, such as is typically used in SCR systems as the reducing agent, has a freezing point of xe2x88x9211xc2x0 C. Here, the volume increases by approximately 10% similarly to the case of water. This increase in volume is compensated mainly by the elastic reducing agent hose, i.e. the rise in pressure is limited by the elasticity of the system. All the components of the system which contain reducing agent must be constructed in such a way that a reduction in pressure in the direction of the hose is possible up to complete freezing. This requires a high degree of resistance to pressure and small dead volumes of the components. In the case of the pressure sensor, the resistance to pressure is limited principally by the pressure sensor diaphragm. The further the burst pressure of the pressure sensor diaphragm lies above the upper measuring limit of the pressure sensor, the lower the sensitivity of the sensor, and thus the measured value resolution and precision. Resistance to frost is therefore most difficult to achieve with the pressure sensor due to the principle.
The increase in volume of water or as here of the aqueous solution of reducing agent at the face transition from fluid to solid is unavoidable. It is not always possible to avoid an uncontrolled rise in pressure in specific system sections here even when using overpressure valves or elastic hose connections, as the freezing largely prevents a hydrostatic pressure equalization.
An embodiment of the invention includes configuring a pressure sensor in such a way that when a fluid freezes, damage to the pressure sensor, in particular to the pressure sensor diaphragm, can be avoided.
The idea on which an embodiment of the invention is based is to make the unavoidable dead volume directly upstream of the pressure sensor element as small as possible and moreover ensure that this dead volume increases elastically in a defined way, and thus limits the pressure on the pressure sensor diaphragm, when an overpressure occurs owing to the phase transition of the fluid.
For this purpose, the pressure sensor element having a pressure sensor diaphragm may be held in a defined position in such a way by way of a spring element while it is operating within an acceptable working pressure range, and a relative movement, acting counter to the spring force, takes place between the pressure sensor element and holding part when the acceptable working pressure range is exceeded. As a result, a volume increase in the space lying directly upstream of the pressure sensor diaphragm is brought about, causing the pressure to be limited. The pressure sensor diaphragm is thus protected against destruction even when the solution of reducing agent freezes, and thus concomitantly by the volume increase in the frozen solution of reducing agent.